This invention applies to the combustor section of gas turbine engines used in powerplants to generate electricity. More specifically, this invention relates to the structure that transfers hot combustion gases from a can-annular combustor to the inlet of the turbine.
In a typical can-annular gas turbine engine, a plurality of combustors are arranged in an annular array about the engine. The combustors receive pressurized air from the engine""s compressor, add fuel to create a fuel/air mixture, and combust that mixture to produce hot gases. The hot gases exiting the combustors are utilized to turn a turbine, which is coupled to a shaft that drives a generator for generating electricity.
The hot gases are transferred from each combustor to the turbine by a transition duct. Due to the position of the combustors relative to the turbine inlet, the transition duct must change cross-sectional shape from a generally cylindrical shape at the combustor exit to a generally rectangular shape at the turbine inlet. In addition the transition duct undergoes a change in radial position, since the combustors are rigidly mounted radially outboard of the turbine.
In a typical gas turbine engine, transition ducts are surrounded by a plenum of compressed air from the engine""s compressor. This air is directed to the combustors and also cools the transition duct walls. Due to the pressure loss associated with the combustion process, the hot gases within the transition duct that enter the turbine are at a lower pressure than the compressed air surrounding the transition ducts. Unless the joints between the transition duct and turbine inlet are properly sealed, excessive amounts of compressed air can leak into the turbine, thereby bypassing the combustor, and resulting in engine performance loss. A variety of seals have been utilized in this region to minimize leakage of compressed air into the turbine. Some examples include stiff xe2x80x9cfloatingxe2x80x9d metal seals, brush seals, and cloth seals, depending on the transition duct aft frame configuration. Most common from a manufacturing and cost perspective are xe2x80x9cfloatingxe2x80x9d metal seals that are manufactured from a formed plate or sheet metal and are installed such that they can xe2x80x9cfloatxe2x80x9d between the aft frame and turbine inlet. Though the xe2x80x9cfloatingxe2x80x9d metal seals are quite common, they still have some shortcomings, such as stiffness and tendency to xe2x80x9clockxe2x80x9d in place. Seals that are too stiff cannot adequately comply with relative thermal growth between the transition duct and turbine inlet. If the seals xe2x80x9clockxe2x80x9d in place they cannot adjust to thermal changes and will leave gaps between the transition duct and turbine inlet. These issues in combination with complex geometry changes, rigid mounting systems, and high operating temperatures as seen by transition ducts create a harsh operating environment that can lead to premature deterioration, requiring repair and replacement of the transition ducts.
To withstand the hot temperatures from combustor gases, transition ducts are typically cooled, usually with air by a variety of methods including internal cooling channels, impingement cooling, or effusion cooling. Severe cracking has occurred with internally air cooled transition ducts having certain geometries that are rigidly mounted to the turbine inlet and contain stiff, rigid seals between the transition duct and turbine inlet. This cracking may be attributable to a variety of factors. Specifically, high steady stresses in the region around the aft end of the transition duct exist where sharp geometry changes occur and a rigid mounting system is utilized. Such a rigid mount located at the transition duct aft end does not allow for adequate movement due to thermal growth of the transition duct. Compounding these problems are stiff xe2x80x9cfloatingxe2x80x9d seals that have a tendency to lock into the turbine inlet during installation, further inhibiting movement of the transition duct aft frame region.
The present invention seeks to overcome the shortfalls described in the prior art by specifically addressing the issues with the rigid sealing system by providing an improved sealing system with increased flexibility, cooling, and leakage control. A sealing system must be able to conform to the turbine inlet during installation and provide an effective sealing mechanism despite the varying thermal gradients between the transition duct aft frame and turbine inlet, while not inhibiting movement of the transition duct aft frame. What is needed is a more compliant metal seal that provides improved flexibility during transition duct installation, effective sealing during all operating conditions, and improved durability under high temperature and vibratory conditions. It will become apparent from the following discussion that the present invention overcomes the shortcomings of the prior art and fulfills the need for an improved transition duct to turbine inlet seal.